Re: [PATCH] x86: emit tzcnt unconditionally

[Uros Bizjak <ubizjak at gmail dot com>]

On Mon, Apr 30, 2012 at 10:09 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Fri, Apr 27, 2012 at 3:30 PM, Paolo Bonzini <bonzini@gnu.org> wrote:
>> tzcnt is encoded as "rep;bsf" and unlike lzcnt is a drop-in replacement
>> if we don't care about the flags (it has the same semantics for non-zero
>> values).
>>
>> Since bsf is usually slower, just emit tzcnt unconditionally. ?However,
>> write it as rep;bsf unless -mbmi is in use, to cater for old assemblers.
>
> Please emit "rep;bsf" when optimize_insn_for_speed_p () is true.
>
>> Bootstrapped on a non-BMI x86_64-linux host, regtest in progress.
>> Ok for mainline?
>
> OK with the optimize_insn_for_speed_p conditional.

I have committed similar patch, where we emit bsf when optimizing for
size (saving a whopping one byte) and rep;bsf for !TARGET_BMI. The
same functionality can be added to *ffs<mode>_1, since we don't care
what ends in the register for input operand == 0 (this is the key
difference between tzcnt and bsf).

2012-05-06  Uros Bizjak  <ubizjak@gmail.com>
	    Paolo Bonzini  <bonzini@gnu.org>

	* config/i386/i386.md (ctz<mode>2): Emit rep;bsf even for
	!TARGET_BMI and bsf when optimizing for size.
	(*ffs<mode>_1): Ditto.

Tested on x86_64-pc-linux-gnu {,-m32}, committed to mainline SVN.

Uros.

Attachment: p.diff.txt
Description: Text document